The invention relates to a sliding bearing for an internal combustion engine and particularly for receiving dynamic load, and a sliding bearing structure having this sliding bearing.
Recently, in order to achieve a light-weight design, a housing for mounting a sliding bearing is thinned in thickness or is used an aluminum alloy, and resultantly the housing is reduced in rigidity. Thus, in the housing, more micro repetitive strains than before are apt to occur due to the dynamic load. For example, in a big end portion of and in a main bearing portion of a connecting rod for the internal combustion engine, a light-weight design is adopted for achieving a high-speed, high-temperature, and high load design. Between an inner face of a housing corresponding to these big end portion, the main bearing portion, and a back face of a sliding bearing attached thereto, there occur relative micro collision and micro slip due to the repetitive strains of the housing, with the result that damage is apt to occur due to fretting wear.
To cope with the fretting wear, there have been hitherto known a sliding bearing in which the back face thereof is provided with a coating layer made of a resin such as PTFE superior in lubrication, and another sliding bearing in which the back face thereof is provided with a coating layer formed by plating silver or copper.
In JP-A-61-88020 there is disclosed a sliding bearing provided on the back face thereof with a coating layer made of a copper alloy or a nickel alloy or an aluminum alloy which coating layer has a hardness of 15 to 80 Hv. By this soft metal coating layer, it is said that the occurrence of the fretting wear is restrained.
In JP-A-2-89813 there is disclosed a sliding bearing having a composite plating layer as a coating layer thereof which composite plating layer is formed by precipitating PTFE in the plating layer of Ni or Co. In this sliding bearing, in order to prevent the plating layer from being peeled off from the back metal of the sliding bearing due to the inferior adherence of PTFE, the composite plating layer of both the PTFE and Ni or Co is provided. By preventing the PTFE with a low friction coefficient from being peeled off, it is said that the fretting corrosion resistance can be fairly improved.
In JP-A-6-94036 according to the present applicant is disclosed a sliding bearing having a phosphate coating as a coating layer. This is one in which the fretting resistance is improved by providing the phosphate coating low in friction coefficient.
However, insofar as the soft coating layer disclosed in JP-A-61-88020 made of a metal having a hardness of 15 to 80 Hv is concerned, an adhesion phenomenon is apt to occur between the back face of the bearing and the housing thereof due to insufficient rigidity, therefore the fretting being apt to occur. Thus, it has been impossible to obtain sufficient fretting resistance insofar as only such means as the soft metal coating layer is concerned.
Further, regarding a coating layer made of both of PTFE and a metal such as Ni which coating layer is disclosed in JP-A-2-89813, due to micro friction occurring between the back face of a bearing and the housing thereof, there still remained such a problem as PTFE is apt to be peeled off, so that it was impossible to obtain sufficient fretting resistance.
Further, after repeating the researching of the phosphate coating disclosed in JP-A-6-94036 according to the present applicant, it has found that the provision of the phosphate coating makes the fretting resistance superior but makes the seizure resistance deteriorated due to the low thermal conductivity thereof.
The object of the invention is to obtain a sliding bearing having both superior fretting resistance and superior seizure resistance, in which a phosphate coating is formed only at a portion at which fretting wear is apt to occur, no phosphate coating being provided at another portion other than the first portion, in which another portion a thermally good conductive coating is formed to enhance the heat-dissipating ability thereof.
In accordance with an aspect of the invention, there is provided a sliding bearing comprising a back metal layer having an inner face and a back face, a sliding layer bonded to said inner face of the back metal layer, a phosphate coating provided at a portion of said back face of said back metal layer at which portion fretting wear is apt to occur, and a thermally conductive coating having superior thermal conductivity which thermally conductive coating is provided at another portion of said back face which another portion is provided with no phosphate coating.
As a sliding bearing to which the invention is applied, it is possible to suitably adopt a double-layers-bearing in which a sliding layer of an alloy or a resin is formed on a back metal, or a tri-layers-bearing further provided with an overlay. As the alloy, it is possible to select one of such aluminum-based alloys, copper-based alloys and etc as to have been publicly known hitherto. As the resin, it is possible to suitably select a publicly known one of polyimide resin, polyamide-imide resin and a resin including one of these resin as a main constituent thereof. Further, as the overlay, it is also possible to suitably select a publicly known one of Pb-based alloy, Sn-based alloy and etc.
As the back metal, a cold rolled, soft steel sheet can be used properly, however, without being limited to this kind, it is possible to suitably use any one. Particularly, it is preferred to use a low carbon steel which is readily worked. As the phosphate coating, a usually used representative is one selected from zinc phosphate, manganese phosphate, zinc-calcium phosphate and ferrous phosphate. In these phosphates, the zinc phosphate has acicular crystals stacked obliquely, each of the manganese phosphate and the zinc-calcium phosphate being provided with crystals of a hexahedron, and the ferrous phosphate has columnar crystals of an indefinite shape.
Further, regarding the thickness of the phosphate coating, it is preferable to provide a phosphate coating of an about 2 to 30 xcexcm in total in thickness on the inner face of the housing and/or on the back face of the back metal. In a case where the thickness of the phosphate coating is less than 2 xcexcm, the wear resistance thereof becomes insufficient. In another case where the thickness thereof exceeds 30 xcexcm, the phosphate coating is apt to be peeled off. Particularly, the thickness value thereof in the range of 4 to 20 xcexcm is preferred in view of the respect of the good durability.
The phosphate coating is formed at a portion where the fretting wear is apt to occur. In explaining the portion in which the fretting wear is apt to occur, this portion means an angular range of the portion xe2x80x9cAxe2x80x9d in FIG. 1 where relative micro collision and/or micro slippage occurs due to the repetitive strains of a housing 2 between the inner face of the housing 2 and the back face of a sliding bearing 1. Namely, regarding the maximum load position xe2x80x9cBxe2x80x9d (which is, in FIG. 1, assumed to be the lowest position), the fretting wear is apt to occur most intensively at two portions each ranging xc2x110 degrees from each of radial lines C which are circumferentially spaced by 45 degrees clockwise and counter-clockwise from the maximum load position xe2x80x9cBxe2x80x9d. Thus, it is necessary to provide the phosphate coating at each of the two portions each circumferentially ranging from the radial line C to xc2x110 degrees. Further, the broader the range of the phosphate coating circumferentially provided becomes, the more the lowering of the heat-dispersing property becomes.
Further, on another portion of the back face other than the above portion at which the phosphate coating is provided, a coating of a material having a thermally good conductivity is formed so that seizure resistance is further enhanced because of the acceleration of heat dispersion achieved by the thermally good conductive coating.
As the thermally good conductive coating, it is possible to use a metal usually represented by copper, silver, nickel, aluminum and etc.
In a case where a sliding bearing structure is formed by combining two half-bearings (, a term xe2x80x9ca half-bearing xe2x80x9d being used when a single piece is meant,), the phosphate coating may be formed on the whole of the back phosphate coating may be formed on another half-bearing located at a non-load side. According to this, the phosphate coating may be formed on the whole back face regarding one of the half-bearings, and on another half-bearing is needed no phosphate coating treatment, so that the production of a sliding bearing can be performed readily.
Further, at the time of the production, it is possible to form a coating of a thermally good conductive material such as copper on a half-bearing located at a non-load side, whereby it becomes possible to obtain a bearing structure having further improved seizure resistance in comparison with that of the above case of no phosphate coating on the non-load side.
As regards the phosphate coating, it may be formed either one of the inner face of the housing and the back face of the back metal layer of a sliding bearing, or may be formed on both of them.
In another aspect of the invention, the phosphate coating is formed on a portion of the back face of a sliding bearing at which portion the fretting wear is apt to occur, and a coating of a material having thermally good conductivity is formed on another back face portion where no phosphate coating is formed, whereby it becomes possible to produce a sliding bearing structure having both superior fretting resistance and superior seizure resistance.
In accordance with still another aspect of the invention, there is provided a sliding bearing structure in which two half-bearings each having a semi-circular shape is combined, in which structure a coating of a material having thermally good conductivity is formed on the whole face of the back face of one half-bearing located at a load side, and in which structure no phosphate coating is formed on another half-bearing located at a non-load side, whereby it becomes possible to produce a sliding bearing structure having both superior fretting resistance and superior seizure resistance.